High-power underwater electroacoustic transducer for the infrasonic and low audio-frequency range

ABSTRACT

This disclosure is directed to a high power underwater electroacoustic transducer operable in the infrasonic and low audiofrequency range.

1101M States Pafienfi 1 1 1111 3,760,346

Kieser et al. 1 Sept. 18, 1973 [5 HIGH-POWER UNDERWATER 1,808,149 6/1931Smith 340 12 R ELECTROACOUSTIC TRANSDUCER FOR 3,823,282 13:32; 3611mmgig/1:31 g

, a meyer THE INFRASONIC AND Low 3,345,607 /1967 Nelkin et al. 340/8 PCA[mm-FRI)UENCY RANGE 2,404,360 7 1946 Bostwick 340 14 2,490,595 12 1949Merten 340 13 R Invfimors' 5E123; 3} 6323i; 0 2,429,104 10/1947 OISOn340/13 R Fla 3,394,775 7/1968 C016 et al 340 12 R [73] Assignee: TheUnited States of America as represented by the Secretary of the PrimaryExaminerBenjamin A. Borchelt Navy Assistant ExaminerH. J; Tudor ,7 t l.Filed: y 1972 Attorney R S Sc1asc1ae a [2]] App]. No.: 251,410

[57] ABSTRACT [52] U.S. Cl. 340/8 R, 340/17 1 This disclosure isdirected to a high power underwater e 0 care electro-acoustic transduceroperable in the infrasonic and low audlo-frequency range.

[56] References Cited 7 UNITED STATES PATENTS 2 Cl 6 D 3,113,125 1 1964Sims 340/8 rawmg gums 111 5G 55 H3 o 43 a2 35 22 I06 as B6 92 as 1 5 z t4 23 B4 L 71 3 i 2 H I 73 15 q 7B 75 9 99 52 a1 17 14 1e PATENTEU SEP181975 SHEET 2 [if d PATENTEI] SEP1 8I975 SHEEI 30F 4 PATENTED 3.760. 346

SHEET u n; a

FIG. 6.

HIGH-POWER UNDERWATER ELECTROACOUSTIC TRANSDUCER FOR THE INFRASONIC ANDLOW AUDIO-FREQUENCY RANGE BACKGROUND OF THE INVENTION This invention isdirected to underwater transducers and more particularly to anelectro-acoustic transducerv operable in the infrasonic and lowaudio-frequency range.

Heretofore, transducers have been provided to meet specific needs; thatis, specific transducers have been used for specific ranges of operationthereby requiring many different types of transducers to cover a widefrequency range. Prior art transducers have inherent limitations such asnarrow range of frequency response, low maximum power input level, lowavailable sound pressure level, fragile moving parts, limited depthcapability, and large size and massive construction. Further, theyrequire considerable effort and time for disassembly for repairs orreplacement of parts.

SUMMARY OF THE INVENTION The transducer made in accordance with theteaching of the present invention is small in size, operable in shallowor deep water over a wide range of frequencies. The transducer includesin combination a magnet mass and a moving coil. A movable piston havingthe moving coil/coil form secured thereto is secured in position bycompliant material and is provided with a piston centering pin coaxialwith the magnet which keeps the moving coil centered in the magnetic gapthroughout the entire length of the piston displacement. Suitable stopsare provided to limit the travel of the piston. The piston is surroundedwith a suitable oil or other such fluid which transmits motion to or forthe surrounding water.

STATEMENT OF THE OBJECTS It is therefore an object of the presentinvention to provide a transducer which has a high acoustic output overa broad low frequency band.

Another object is to provide a transducer in which the output is nearlylinear with input over a broad frequency band.

Yet another object is to provide a transducer which is easily assembledand disassembled for replacement of parts or to make repairs.

Still another object is to provide a transducer which is sufficientlysmall that a family of transducers, each of which have differentcharacteristics, may be assembled into a unitary structure to provide avery wide range of electroacoustic characteristics.

While still another object is to provide a transducer assembly whichincludes an efficient cooling system which allows high driving currents.

These and other objects will be better understood with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cut-away viewillustrating the relative parts of a preferred transducer.

FIG. 2 illustrates a front view of the back pole piece.

FIG. 3 is a cross sectional view of FIG. 2 taken along lines 3-3.

FIG. 4 is a front view of the outer front pole piece.

FIG. 5 is a cross sectional view of the outer front pole piece.

FIG. 6 is a partial view illustrating a modification of the device shownin FIG. ll.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawing,there is shown by illustration in FIG. 1 a transducer element made inaccordance with the teaching of the present invention. The transducerelement includes a centrally located, grain oriented ALNICO V-7, dualdiameter magnet 10 formed by solid sections 11 and 12 provided with anuniform coaxial aperture 13' therethrough. The magnet section 12 isprovided with a cutout about the aperture to form a shoulder 14. Acylindrical magnetic ingot iron front pole piece 15 is secured to thefront face of section 12 and is provided with a section 16 that fitsinto the cutout in the face of section 12 and rests upon the shoulder14. The front pole piece has a central aperture colinear with theaperture in the magnet and is cutout on the front face about theaperture to provide a shoulder 17. The outer diameter of the front polepiece is the same as that of the magnet pole section 112.

A soft iron back pole piece 18 having a copper coating of LOOD ZSinchthereon is secured to the back face of magnet section 11. The back polepiece has a central aperture in alignment with the aperture in themagnet with the aperture 19 increasing toward the back on the radius ofa circle which is less than the thickness of the pole piece. The backpole piece has a section 211 at a 45 angle which extends to a cylindrialsection 22 that extends toward the front surrounding the magnet 10 andis spaced therefrom. The front face of cylindrical section 22 is on thesame plane as the front face of section 12 of the magnet. Thecylindrical section is provided with two equally spaced axially alignedapertures 23 therethrough through which electrical conductors are passedand eight apertures 24 through which bolts pass to secure the pole piecein place. Eight holes 25 are drilled near the front end perpendicular tothe axis through which fluid may flow from one side of the magnet to theother, as shown in FIGS. 2 and 3. The 45 angle section is provided withoppositely disposed apertures 26 within which electrical terminals aresecured to provide an electrical connection between a charging coil 27in the spacing betweenthe magnet and the cylindrical portion of the backpole piece. The magnetic back pole piece is further provided withsuitable threaded holes 28 and 29 on the front and back sidesrespectively, for securing other elements to the magnetic pole piece.

The spacing between the magnet 10 and the cylindrical portion of theback pole piece 22 includes therein the charging coil 27 formed byapproximately 51 turns of No. 13 A.W.G. type KZSFX heavy insulationclass 200 magnetic wire. The coil is shown formed by three coils oneover the other each having an 0.015 inch fibre insulation therebetween.The ends of the coil are secured to suitable terminals secured in theapertures 26 through the back pole piece. The remaining spacing betweenthe magnet 10 and the cylindrical'portion of the and back pole piece areproperly aligned. The centering ring further serves to make the unitmore rugged, and the lead-epoxy potting compound damps spuriousresonances and increases the mass of the static system. The centeringring 31 is also provided with cutouts that align with the apertures 25in the back pole piece to aid in allowing the fluid flow through theapertures 25.

The cylindrical section of the back pole piece has secured thereto anouter front pole piece 32 by use of suitable bolts which pass throughapertures 24 in the back pole piece and are threaded into appropriatethreaded holes in the outer front pole piece. The outer front pole pieceis of cylindrical shape and surrounds the inner front pole piece 15secured onto the magnet 10. The inner front face 33 of the outer frontpole piece as shown in FIGS. and 6 has a flat surface which is parallelwith the surface of the inner front pole piece and spaced therefrom. Theback face of the outer front pole piece has a circular groove 34 thereinwhich receives an o-ring 35 and a circular protrusion 36 whose diameteris slightly less than the inner diameter of the cylindrical portion 22of the back pole piece such that the end of the cylindrical portion ofthe back pole piece just slips over the protrusion when assembled. The

outer surface of the outer front pole piece is in the form of steps 37and 38 such that portions of the element is of greater diameter than theothers. The step 37 has notches 41 therein and the steps 38 is providedwith a groove 42 within which an o-ring seal 43 is placed. The elementis provided with angularly drilled apertures 44 which extends outwardtoward the back to meet with the parallel apertures 23 in the back polepiece through which electrical conductors 45 are passed to an electricalterminal 46 secured within the aperture 44 as shown in FIGS. 1 and 6. Aplurality of axially aligned parallel apertures 47 extend through theelement near the inner surface through which fluid may flow from thefront of the element to an area to the back thereof. An aperture may bedrilled through the element to the back side thereof through which aheat pipe may be passed to aid in cooling the transducer. The coolingsystem is not shown for clarification of the drawings. The back side ofthe element is provided with threaded holes 51 between the groove 34 andthe protrusion 36 to which an oil seal ring 52 is secured and threadedholes 53 outwardly of the protrusion by which the outer front pole pieceis secured to the back pole piece by suitable bolts that pass throughapertures 24 in the back pole piece. The portion of the element that hasthe greatest diameter has threaded holes 54 therein directed from thefront surface by which the magnet assembly is secured within the housingby use of a cylindrical compression gasket 55 which is forced by aplurality of bolts 56 between the housing and the step 37 of the outerfront pole piece. The front face is also provided with threaded holes 50for securing a piston protection cage and holes 60 for securing anacoustic window assembly.

A piston 61 operated by a voice coil 62 positioned between the frontouter and inner pole pieces is secured in place by a single ring-likecompliant member 63. The compliant member is vulcanized to the brass oilseal ring 52 which is provided with a V-groove 64 for reception of theouter edge of the compliant member. The brass ring is provided with aplurality of apertures 65 through which screws 66 pass to secure thebrass ring to the back surface of the outer front pole piece against theo-ring 35 placed in the groove within the surface of the outer frontpole piece. The opposite end of the compliant member is secured to oneend of an aluminum coil form 68 which is provided with a cutout section71 about which the voice coil 62 is wound. The opposite end of the voicecoil form is secured to the piston.

The voice coil comprises 208 turns of 30 AWG type KZSFX singleinsulation class 200 magnetic wire 69 wound in four rows or layers offifty two turns each. The voice coil form 68 is placed on a mandrel, acoating of high temperature epoxy adhesive is applied to the surface ofthe form about which the coil is to be wound. The first layer is wound,then a second coat of epoxy adhesive is applied, etc, until the fourlayers are wound on the coil form and the outer layer coated. Whilerotating the finally wound coil, apply some heat such as a sun lamp toset the epoxy adhesive. After the adhesive has set, the coil may beheated in an oven to cure the coating. The coil form is provided withoppositely disposed grooves 70 along the length of the coil form towardthe front within which the wire leads lay in order to conect the wireends to electrical terminals secured to the piston without extendingpast the outer radius of the coil form.

The piston is formed by a cylindrical section 71 and a closed end noseportion 72 that is perpendicular to the axis of the device joinedtogether by a conical section 73. The nose is provided with an axiallyaligned cylindrical hollow piston centering pin 74 which extends towardthe aperture through the magnet. The front inner pole piece on themagnet has an axially aligned cylindrical piston guide 75 securedthereto coaxial with the piston. The piston guide is provided with anaperture through which the piston centering pin 74 passes and apertures77 in the wall thereof through which fluid may flow. A portion of theinner surface of the piston guide is provided with a teflon bushing 78through which the piston centering pin 74 operates. The bushing is heldin place by a set screw 79. The bushing and a chromium plated sleeve 81on the outside of the piston centering pin reduces friction duringmovement of the piston. A compliant, piston limiting stop 82 is securedto the end of the piston guide to limit the inward piston travel. Theoutward travel is limited by a stop 83 on a support cage 84 outwardly ofthe piston. The support cage is secured by suitable screws 76 to theouter front pole piece which thread into threaded holes 50. Movement ofthe piston is controlled by an electrical current which is applied tothe voice coil thereby producing a magnetic reaction between the voicecoil and the permanent magnet assembly.

The device operates in an oil or other suitable fluid filling which bothcools and transmits the effects of sound waves between the surroundingmedium and the piston. The oil is contained by end brass cylinder 86 anda rubber front window 87 secured thereto by any suitable method such asby vulcanization. The brass cylinder is provided with an outwardlyextending flange 88 which is provided with a plurality of aperturestherethrough by which it is secured to the outer front pole piece by useof suitable screws. The flange extends outwardly even with the stepportion 37 on the outer front pole piece.

For protection of the window, a cage formed of 3/16 inch rod 89 weldedto a ring 90, may be secured in place to the outer front pole pieceoutwardly of the end brass cylinder. As shown, the outer front polepiece and magnetic assembly are secured within the housing by use of thecompression gaskets 55 which are compressed by bolts 56 which arethreaded into the holes 54 in the outer front pole piece. Tightening ofthe bolts forces the compression gasket outwardly against the inner wallof the housing against the outer face of flange 88 and a portion of theouter front pole piece thereby holding the assembly in the housing.

The end brass cylinder 86 is provided with a threaded aperture 91therein through which oil may be poured to fill the enclosed area. Aplug 92 is used to plug the aperture 91. The oil may flow into the areabehind the outer front pole piece through the small apertures 47 boredthrough the outer front pole piece and the magnet gap within which thepiston moving coil is positioned. The apertures and the magnetic gap arekept small to keep acoustic energy from being shunted to the rear of thefront pole piece.

The central aperture through the magnet is closed at both ends byacoustic resistors 96 and 97 and by diffusion plates 98 and 99, in orderto minimize unwanted air-cavity resonances. The sonic resistors are madeof a stainless steel fiber metal manufactured by Huyck Metal Company,Milford, Conn. This material permits air to filter through from one sideof the magnet assembly to the other. The diffusion plates are spacedfrom the acoustic plates 96 and 97 by a spacer 100 which permits air toflow between the acoustic-plate and the diffusion plate. The diffusionplate minimizes interference from the housing cavity.

The back end of the transducer housing is closed by a brass plate 101which is secured in the end of the housing by screws 102 around theoutside of the housing. The brass plate is provided with oppositelydisposed apertures 103 near the housing wall to permit water to enterthe area between the plate and a compliant pressure compensating bag104. The compensating bag is secured about a plate extension 105 by useof a pressure band 106 or any other suitable means. An 0- ring 107 isplaced between the back plate and the inner surface of the housing toprevent water leakage into the inner area of the housing.

The housing is provided with a water proof electrical connector 111through which the required electrical conductors are brought andconnected to an electrical terminal 112. The housing is also providedwith a valve 113 through which air or other fluid may be added to theback side of the magnet for equalization of the inner and outerpressures during use.

All areas in which o-ring seals should be used to prevent fluid leakagefrom one area to another have not been particularly set forth. it isconsidered to be within the skill of one in the art to place an o-ringor other type seal where necessary without resorting to invention.Therefore, one should place an o-ring or other seals where necessaryeven though they have not been specifically setv forth in thespecification. The drawings are believed to adequately illustrate theuse of such 0- rings.

FIG. 6 illustrates a modification of the device shown in FIG. I. it hasbeen determined, if desired, that instead of using the front window andsidewall cavity or brass cylinder, the corrugated rubber front windowmay be molded directly to the outer surface of the piston. in thismodification, a short cylinder 121 is secured in place by thecompression gasket and a corrugated rubber ring 122 is secured to thepiston and to the short cylinder. Since the brass cylinder has beenremoved, the pipes, if used, as set forth above for permitting fluidflow from behind the voice coil to the sump will not be used. However,fluid may flow through the passages in outer front pole piece. Since thepiston will be exposed to the surrounding medium, heat from the voicecoil will be conducted to the surrounding medium through the piston.

in assembly of the various parts of the transducer, the various partsare formed, prepared by drilling apertures therethrough or threadedholes therein in their proper relationships such that they may beassembled together in their proper places. Prior to assembly of thetransducer elements, the housing is first prepared by connecting theelectrical connector, the cable, and terminal in place as well as thefluid valve. The magnet, pole pieces, piston, etc. are assembledtogether and then secured into the housing by use of the compressiongasket. The soft iron back and inner front pole pieces are secured tothe back and front faces of the dual diameter magnet, respectively. Thecharging coil is wound about the magnet in the space between the magnetand the back pole piece that surrounds the magnet and the wire ends arebrought out through apertures in the back pole piece and connected tothe electrical connectors secured thereto. The lead-epoxy mixture ispoured into the spacing to fill the space in the area of the chargingcoil, and the centering ring is positioned in place in order to securethe magnet coaxially within the soft iron back pole piece.

The centering guide including the bushing, piston limit-stop, sonicresistor, and diffusion plate is secured in place onto the inner frontpole piece.

The oil seal ring, compliant piston diaphragm and coil form areassembled, and the coil form is cemented to the piston. The voice coilis then wound onto the coil form and cured. The wire ends from the voicecoil are connected to the electrical connectors secured onto the piston.Thus, the coil, coil wire, and piston will move together as one unit.The heat pipes, if used, may then be assembled onto the back side of theouter front pole piece. The coil-piston assembly is then secured to theback side of the outer front pole piece. The outer front pole piece isthen secured to the back pole piece by bolts that pass through the backpole piece and threaded into threaded holes in the outer front polepiece. During this assembly the piston centering pin is inserted intothe bushing on the front of the inner front pole piece. The electricalwire for the voice coil is passed through the aligned apertures in theback pole piece and the outer front pole piece and secured to theelectrical connectors which are then secured in the apertures in theouter front pole piece. These wire conductors are long enough to connectto the supply terminal when assembled in the housing. A wire from theelectrical connector in the outer front pole piece is then connected toeach electrical terminal secured to the piston.

The piston cage used to limit the outer movement of the piston may besecured to the front side of the outer front pole piece. The magnet isthen magnetized. The brass cylinder to which the rubber front window isattached is then connected to the front outer pole piece by sufficientbolts or screws, and filled with some appropriate liquid such as castoroil. The compression gasket and studs therethrough may then be securedin place. The compression gasket is not compressed until the magnetassembly is inserted into the housing. The four equally spaced isolationspacers may be secured to the shouder on the inner surface of thehousing. The magnet unit is now ready to be placed into the housing.Thus, the assembled element is slid into the housing until the back faceof the outer front pole piece contacts the isolation spacers. Theprotective cage is then placed over the studs through the compressiongasket, nuts are placed thereon and then tightened in order to securethe assembled element into the housing by compressing the compressiongasket. The wires from the voice coil may then be secured to theelectrical terminal on the housing through the back open end of thehousing.

Prior to assembly of the magnet into the assembled element, the magnetis not magnetized. If the magnet element is first magnetized, it wouldbe almost impossible to assemble the various parts and insert theassembled unit into the housing. Therefore, in order to properly alignthe various parts during assembly, the permanent magnet is magnetizedafter assembly into the element. Thus, the requirement for the chargingcoil. While in the assembly shop, the charging coil wires are connectedto a charging source, the magnet charged, and then disconnected once themagnet is permanently magnetized. A suitable handy charging source hasbeen found to be a 24 volt D.C. battery bank which supplies 300 Amps forseveral seconds. The charging coil only serves to charge and dischargethe magnet and is not used during operation of the device.

Subsequent to securing the element in the housing and connecting theelectrical leads in place, the back plate and compensating bag aresecured in place, and the desired fluid is added to the area between themagnet and the compensating bag at atmospheric pressure.

In operation, the permanent magnet sets up a magnetic field through theback and front pole pieces thereby producing a magnetic flux across thegap between the inner end of the outer front pole piece and the innerfront pole piece. An A.C. voice current is applied through the voicecoil to produce an alternating flux about the voice coil. Theinteraction between the A.C. produced magnetic flux and the permanetmagnetic field causes the piston to move axially. The amount of movementis proportional to the current in the voice coil. Movement of the pistonapplies a force on the fluid that surrounds the piston which applies aforce on the rubber front window and the surrounding water. The pistonmovement transmits pressure waves in the surrounding water which may bedetected by suitable equipment. The front window being compliant permitslarge piston displacement, yet it maintains a low fundamental resonance.

The transducer may be used as a single unit or it may be assembledside-by-side with two, three or more of like units to improve theoutput.

Obviouly many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of theUnited States is:

l. An improved transducer element including a fluid tight housingenclosed by a compliant front window and a compliant compensating bag inthe back area thereof with a magnet element fixed in the housing,between said front window and said compensating bag, the improvementcomprising,

a compliant support member secured to said magnet element,

a voice coil assembly within said housing secured to said compliantsupport member at its rearward end for axial movement relative to saidmagnet elemerit,

a piston secured to the front end of said voice coil assembly,

a centering pin in combination with said piston extending along the axisoperative to prevent off-axis movement of said piston and said voicecoil during axial movement, and

a permanent magnet charging coil wound about a central portion of saidmagnet element for permanently magnetizing said magnet elementsubsequent to assembly into said housing.

2. An improved transducer as claimed in claim 1;

wherein,

said magnet includes an outer front pole piece with an inner front facefacing the axis of said housing,

said voice coil assembled adjacent the front face of said front polepiece, and

said outer front pole piece includes apertures therethrough adjacentsaid voice coil to permit fluid flow to the back of said outer frontpole piece and surrounding said voice coil for cooling the magnet.

1. An improved transducer element including a fluid tight housingenclosed by a compliant front window and a compliant compensating bag inthe back area thereof with a magnet element fixed in the housing,between said front window and said compensating bag, the improvementcomprising, a compliant support member secured to said magnet element, avoice coil assembly within said housing secured to said compliantsupport member at its rearward end for axial movement relative to saidmagnet element, a piston secured to the front end of said voice coilassembly, a centering pin in combination with said piston extendingalong the axis operative to prevent off-axis movement of said piston andsaid voice coil during axial movement, and a permanent magnet''''charging'''' coil wound about a central portion of said magnetelement for permanently magnetizing said magnet element subsequent toassembly into said housing.
 2. An improved transducer as claimed inclaim 1; wherein, said magnet includes an outer front pole piece with aninner front face facing the axis of said housing, said voice coilassembled adjacent the front face of said front pole piece, and saidouter front pole piece includes apertures therethrough adjacent saidvoice coil to permit fluid flow to the back of said outer front polepiece and surrounding said voice coil for cooling the magnet.